BRUEL & KJAER 2631 User guide
Microphone Carrl•r System
Typa
2631
Microphone Carrier System
Type
2631
A carrier system designed
for
detection of capacitance variations
in the frequency range
DC
to
150 kHz. Used with a condenser
microphone input it provides
an
ideal first stage processing of low
frequency pressure variations such
as sonic bangs, thunder storms,
short duration pulses etc.
MICROPHONE CARRIER SYSTEM
TYPE 2631
December 1969
CONTENTS
1.
INTRODUCTION . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.
DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Microphone
Head
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Control
Unit
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.
OPERATION . . . . . . • . . . . . . • . . . . . . . . • . . . . . . . . . . . . . . . . . 9
Input
..........................................
10
Outputs
.....
.
................
.
.................
11
Lower
Limiting
Frequency
.........................
11
DC
Balance
.....................................
12
Gain Adjustment
.................................
12
Test Oscillator
..........
.
...............
0
••
0 • • • • •
13
Power
...........
.
...
0
••••
0.
0
•••
0.
0
••
0.
0 • •
••••
• 14
Removal
of
Module
Panels
..........................
14
Microphone
.....................................
15
Calibration
......................................
15
4.
APPLICATIONS
•.......•............•.•.....•........
18
Sonic
Bang
Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Measuring Environment
............................
18
Microphone
and
Installation
........................
18
Extension
of
Microphone lower
limiting
Frequency
......
19
Measurements
and
Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 19
Pressure
Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5, SPECIFICATIONS
.••.•...............................
26
1.
INTRODUCTION
The
B&K
Microphone Carrier System Type 2631
has
been
developed
to
meet the exacting requirements called
for
in the measurement
of
low fre-
quency
pressure
variations down
to
0.01 Hz,
for
which special measurement
techniques
are
required.
The
basic
2631 Carrier System
is
composed
of
a
Head
and a Control
Unit
containing the power supply
and
output
amplifier. A condenser microphone
(not provided)
can
be
plugged directly into the
Head.
Any
pressure
differen-
ces
present
at
the microphone's diaphragm
are
converted
to
capacitance
variations which
are
detected by
an
FM
detector contained in the
Head.
The
2631
has
adynamic
range
of
95 dB
("A"
Weighting)
and
unlikemost conven-
tional systems the demodulated
output
from
the
head
allows the possibility
of
reasonably long connecting
cables
to
the control unit.
In order
to
realise the very
low
frequency
response,
special
condenser
microphones
are
employed in which the
pressure
equalisation hole
is
sealed,
thus bringing the lower frequency
limit
down from 3 Hz
to
below
0.1
Hz.
However, given
such
a microphone there would
be
considerable
difficulty
in
choosing a cathode follower and preamplifier
as
used
in conventional
sy-
stems.
The
input
impedance required would
be
in order
of
3.
101
0 ohms
which would
be
exceedingly
difficult
to
realise in conditions
of
high humidi-
ty.
The 2631
has
the advantage that its
input
impedance.
is
below 100 ohms
and
therefore
not
affected
by
moisture.
In the
DC
mode adjustment
is
provided
for
manual balancing
and
compensation
of
the system
for
·different transducer capacitances.
Balance
is
clearly indicated on a monitoring meter. Balancing in the AC mode
is
auto-
matic.
The special
measi,Jrement
techniques called
for
in the analysis
of
slowly
varying air
pressures
and
short duration
pressure
pulses
such
as
found in
sonic
bangs,
fluid
mechanics, wind tunnels,
pressure
chambers
and
thunder
storms make the
B&K
Carrier System eminently suitable
for
such
applica-
tions.
3
dB
0
-20
-4-0
-60
p
Rel.Amplitude
Freq.
.001 .01
.I
I
10
100Hz
Hz
~,8/~:t
Fig.
1.
1 Idealised N
wave
of
sonic
bang.
-p~--~----------------------
-
t
~I
Tt'me
Fig.1.2 Fourierspectrum
of
sonicbani.
4
Taking for example
the
idealised N wave
of
the
sonic bang from an
aircraft (Fig.1.1) ,
the
peak overpressure, p1 may be 5 lb/ft2 (250 Nfm2)
or
possibly even higher for low flying aircraft, and
the
pulse duration,
t,
of
the
order 0.03
to
0.4
sec. A Fourier integral of such an N wave shows
that
its
spectral
content
(Fig.1.2) peaks
at
about
1
/t
Hz
and ranges several decades
above and below this frequency. For accurate representation
of
such spectra
it
is
-necessary
to
have a measurement capability from 0.01
Hz
to
10 kHz as
recommended by
the
I.S.O. Draft Proposal 43/1 N 11. The frequency range
of
the
2631 Carrier System extends from
DC
to
150
kHz and
is
thus ideally
suited
to
this and
other
applications.
5
2.
DESCRIPTION
GENERAL.
A block diagram
of
the
B&K
Type
2631 Microphone Carrier System
is
shown
in
Fig.2.1. A detailed description
of
the
systems design follows
below.
Oocillator
Oetoctor
Proamplifior C8ble Test oscillator Miller
integrator
I I Output
amplifier
I I
Low.r
l•m
Freq.
wlector
Microphone
Head
Control
Unit
Fig.2. 1 Block diagram
of
the Microphone Carrier System.
MICROPHONE HEAD.
The
condenser microphone,
not
provided with
the
2631 can be plugged
straight into
the
Microphone Head and will
act
as
the
detecting transducer,
the
variable capacitance
of
which forms an integral
part
of
the
Head's reso-
nant
circuit.
This resonant circuit
is
driven by a
10
M Hz, crystal controlled oscillator,
and as
the
capacitance
of
the
condenser
microphone
is
varied by sound
pressure variations
at
its diaphragm so
the
impedance
of
the
resonant circuit
is
coupled inductively
to
a circuit which
detects
the
impedance changes, and
produces a
proportionate
output
voltage.
The
demodulated
signal
is
then
amplified by an integrated circuit preamplifier and fed by cable
to
the
Control Unit containing
the
output
amplifier.
6
As
the
demodulated
signal
is
taken from
the
Head and its
output
impe-
dance
is
below 1
00
n,
the
effect
of
cable capacity
on
the
response
of
the
system
is
considerably reduced. See Fig.3.3. Thus any reasonable length
of
cable can be used
to
connect
the
output
from
the
Head
to
the
Control Unit.
The
polarity
of
the
system
is
such
that
an increase
in
capacitance
at
the
input, or an increase
in
pressure
at
the
microphone diaphragm produces an
increase in
output
voltage.
When
in
the
AC
mode compensation
is
obtained for
the
effects
of
diffe-
rent microphone capacitances and thermal variations
of
the
capacitance
diodes
in
the
resonant circuit, by applying voltage feed back from
the
Control Unit's
output
amplifier
to
the
variable capacitance diodes. This
ensures
that
the
resonant circuit
is
automatically
tuned
for transducer capa-
citances between
40
and
70
pF. This
is
adequate for any
of
the
B&K
1 inch
Condenser Microphones, however, for
1/2
inch Microphones a special adap-
tor
Type
UA
0271
is
r~quired.
CONTROL UNIT.
The
demodulated signal from
the
Microphone Head
is
fed
to
one
of
the
inputs
of
the
Control Unit's integrated, differential
output
amplifier.
Voltage
feed-back
is
applied from
the
output
to
the
other
input of
the
amplifier.
The
amount
of
feed back applied determines
the
gain which can
be manually adjusted by
the
Gain Adj.
potentiometer
control.
The
Control Unit's low impedance
output
(<
10
n)
is
suitable for con-
nection
to
practically
any
type
of
recording or· measuring equipment. Out-
put
voltage
is
up
to
12
V,
10
mA depending on
the
setting
of
the Gain Adj.
control.
The
Lower Limiting Frequency
is
controlled by
the
output
amplifier'sfeed-
back loop
by
means
of
a Miller integrator with switchable time constants.
The
feedback signal
is
then
applied
to
capacitance diodes
in
the
resonant
circuit
in
the
Head.
Automatic
compensation with a preselected time con-
stant
is
thus
provided
for
different transd·ucers
in
the
AC
modes.
In
the
DC
mode, compensation
is
set manually
by
the
DC
BALANCE control on
the
front
panel. Balance and overload indication
is
provided
by
the
monitoring
meter, also
on
the
front
of
the
equipment.
In
the
AC
mode
the
response
of
the
Carrier System (without microphone
Fig.3.4) falls. by approximately 1.5 dB
at
the
LOWER LIMITING FRE-
QUENCY selected. It should be noted
that
the
selected frequency
is
not
the
7
cut
off
frequency
(-3
dB point). However,
if
a microphone
is
used
that
has
the
same
lower
limiting
frequency response
as
that
selected on the 2631,
then this would become the
cut
off
frequency. The response
of
the Carrier
System in the
DC
mode
is
that
of
the microphone
used.
Without
micro-
phone the response extends
from
DC
to
150kHz.
A separate test oscillator
is
inductively coupled
into
the feed back loop
after the Miller Integrator and injects a 1 kHz signal
into
the Head's
reso-
nant circuit. Here the 1 kHz signal
puUs
the capacitance diodes producing a
capacitance variation. The variation
is
detected and amplified, and thus
provides a test
for
the whole 2631 Carrier System.
Also contained in the control
unit
is
a stabilized power supply which
provides +15 and
-15
Volts
DC.
for
circuitry
contained in
both
the
Head
and
the Control Unit. The supply
is
mains driven and the approximate
current consumption
is
20
mA
and independent
of
voltage.
8
3.
OPERATION.
The
ease
of
operation
of
the
B&K
MicrophoneCarrier System
Type
2631
can be
attributed
to
the
fact
that
all controls,
input
and
output
sockets etc.,
are confined
to
the
FRONT and REAR PANELS (Fig.3.1 and 3.2 respec-
tively)
of
the
Control Unit.
The
unit's
top,
bOttom and side panels are
Fig.3.1
Front
panel.
9
Fig.3.2
Rear
panel.
readily removed enabling quick
adjustment
of
the
system's few internal
preset controls.
The
operating and calibrating procedure, plus
the
range
of
suitable
B&K
condenser microphones
for
the
Carrier System, follow
in
this
chapter.
INPUT.
INPUT
to
the
Control Unit from
the
Head
is
via a standard·
B&K
7 pin
microphone
socket
on
the
front
panel.
The
socket
also provides
the
Head's
supply and feed back voltages.
10
OUTPUTS.
OUTPUT from
the
Control Unit
is
either
by
a standard
B&K
coaxial socket
on
the
front
panel or by a
10-32
NF
unified socket
at
the
rear.
The
outputs
impedance
is
less
than
1on and
is
suitable for connection
to
practically any
type
of
recording
of
measuring equipment.
The
response
of
the
carrier
system with capacitive loading
is
shown
in
Fig.3.3.
Note
that
a
DC
voltage
of
up
to
12 V can exist
at
the
OUTPUT if
the
system
is
not
balanced and
is
in
the
OC
mode.
10V
7
0,1
0,01
0,1
10
Fig.3.3
Response
of
carriersystem
for
capacitive loading
of
Control
Units output.
LOWER
LIMITING
FREQUENCY.
The LOWER
LIM.
FREQ. can be set
to
1, 0.1, 0.01
Hz
or
to
DC
accor-
ding
to
the
knob position
on
the
front
panel. A further switch position
is
provided for adjustment
of
the
DC
BALANCE. It
is
advisable
to
use
the
11
highest possible setting
of
the
LOWER LIM. FREO. knob if
the
frequency
content
of
the
signal
is
known. This removes undesirable noise
components
below th'at frequency. '
The
Response of
the
lower limiting Frequency
is
shown
in
Fig.3.4.
·
OdB
-1
-2
~3
-4
-5
-6
-7
-8
-9
-10
0.001
I
I
I
I
DC
BALANCE.
.......
~--
~
...
f--
~
v v
.,.
}
Lower
Limiting
Frequency
Umits
I
II
I I
I I
I
I
5
0.01
2 5
0.1
5 10 2 5
100Hz
1'69160
Fig.3.4 Lower
limiting
frequency
response.
When
the
Carrier system
is
operated
in
its
DC
mode,
the
DC
BALANCE
should be set
for
central deflection
on
the
front
panel's monitoring meter.
For
this
purpose
the
LOWER LIM. FREO. switch should be set
in
its
extreme
anti-clockwise
position. Once balance
is
obtained, it should be
returned
to
the
DC
position
for
measurements.
The
DC
BALANCE
knob
controls a single
turn
fine
adjustment
potentiometer
and a
multi-turn
coarse
potentiometer.
DC
balance
of
the
output
in
the
AC
mode
is
automa-
tically adjusted
to
within ±
20
mV,
but
if
it
is
important
that
the
output
signal has no bias
then
further
adjustment
of
the
signal using an external
voltmeter will be necessary.
The
adjustment
potentiometer
is
accessible
by
removing
the
top
panel
of
the
unit
and
adjusting
the
small potentiometer,
shown
in
Fig.3.5., with a screwdriver.
GAIN ADJUSTMENT.
The
gain
of
the
output
amplifier can be adjusted by
the
screwdriver
operated GAIN ADJ.
potentiometer
on
the
front
panel
of
the
unit. This
is
particularly useful
in
~alib~ation
of
the
measuring system as described later..
12
•
Front
Top
view
of
module
with
top
paner
removed.
DC
balance
adjustment
for
AC
modes.
Fig.3.5 Adjustment
of
DC balance in
AC
mode.
TEST OSCILLATOR.
A test oscillator
of
1 kHz frequency
is
included in the
unit
and provides a
simple check on the
function
of
the measuring system. Operation
is
by
depressing the Test
button
on the
front
panel. The amplitude
of
the oscilla-
tor
signal
is
variabl.e and
is
adjusted
by
a
multi-turn
potentiometer
acces-
sible
by
removing the
left
side panel (viewed
from
the rear)
of
the
unit
(see
Fig.3.6). Note
that
since the test oscillator injects a signal
into
the Head, the
output
level
of
the signal
will
also
be
dependent on the
GAl
N
ADJ.
of
the
Carrier System.
13
POWER.
Test
oscillator level
adjustment
potentiometer
Fig.3.6 Adjustment
of
test oscillatorlevel.
The mains input socket together with
the
main power switch are located
at
the
rear
of
the
unit, whilst
the
red mains indication lamp
is
situated
on
the
front
panel. Any mains voltage between
100
and
240
Volts AC,
50
to
400Hz
is
suitable and no adjustment
is
necessary.
The
50
rnA
250
V main
fuse located
at
the
rear
of
the
equipment
is
only accessible when
the
bottom
panel
is
removed.
REMOVAL OF MODULE PANELS.
The BOTTOM PANEL
of
the
2631 Control Unit
is
removed by releasing
the
large coin operated/finger screw
at
the
bottom
of
the
front
panel (see
Fig.3.1.). Quarter
to
half a
turn
should be adequate
to
release
the
catch.
The
panel
is
then
removed by sliding it
towards
the
rear
of
the
unit.
TOP and SIDE PANELS
of
the
instrument can each be released
by
un-
screwing
the
fixing screws on
the
rear
of
the
.unit (see Fig.3.2). These panels
are also removed by sliding
them
backwards.
14
•
•
MICROPHONE.
For low frequency measurements
the
B&K
1 inch Condenser Microphone
Type
4146
is
recommended as its pressure equalisation vent has been sealed
with a silicon rubber ring
as
in
Fig.3.7. This gives a linear frequency re·
sponse which
is
guaranteed from 7 kHz down
to
0.1 Hz. However, this can
be made
to
extend down
to
0.01
Hz
as a proposed system shows
in
the
Applications Chapter.
Fig.3.7
Low
frequency condenser microphone.
B&K
1 inch Microphone Type
4144
and
4145
can also be sealed using
the
Sealing Kit Type
UA
0240. With these microphones,however,
the
0.1 Hz
limiting frequency can
not
be guaranteed.
The
Microphone Head normally accepts a 1 inch condenser microphone,
but
for high frequency, high intensity measurements a
B&K
1/2
inch micro·
phone Type
4148
with an Adaptor
Type
UA 0271 should be used. This
is
especially applicable
in
the
case
of
sonic bangs where a maximum measuring
frequency
of
10kHz
is
normally required.
CALIBRATION.
Ideally
the
complete instrumentation should be calibrated both before
and after measurements. This
is
best achieved by applying an accurately
15
known sound pressure
to
the microphone and adjusting the measuring and
recording instruments
to
indicate correctly. The
B&K
Pistonphone Type
4220 and
the
Sound Level Calibrator Type 4230
are
very convenient
for
this purpose. The Pistonphone produces a sound
pressure
level at the micro-
phone diaphragm
of
124 ±0.2 dB at
250Hz,
while the Sound Level Cali-
brator produces a level
of
94
±0.3 dB at
100Hz.
A
coarse
calibration
utilizing
the sensitivity
of
the Carrier System
with
a
B&K
1 inch Condenser Microphone Type 4146
can
be
applied
if
an
accurate
source
is
not
available. The instrumentation
for
calibration should
be
set
up
as
shown in Fig.3.8., in
an
environment free
from
external pressure distur-
bances.
Balance
is
then obtained
for
the 2631
with
the microphone placed
4146
Microphone
2631
M ic r o
phone
He
a d
Control
Unit
2631
DC
Yo
I
tme
ter
Fig.3.8
Coarse
calibration instrument set up.
in a horizontal position. The
DC
mode
is
then selected and the microphone
(still in a horizontal position) displaced a suitable distance
to
a
point
im-
mediately below the position
for
balance.
To
calibrate, adjust the Gain Adj.
on the Control
Unit
to
give a deflection on a
DC
voltmeter connected
to
the
2631
Output.
The required deflection
for
calibration
can
be
calculated
from:
1.25.
1Q-2
Mp.
d
mV.
Where
"Mp"
is
the
microphone sensitivity (mV
/N/m2)
and
"d"
is
the
distance
the
microphone is displaced (mm).
1"6
.
This represents a pressure difference
of
1 }Jbar/8mm
of
microphone dis-
placement.
Example:
If a Microphone
Type
4146
is
displaced
160
mm below
the
point
at
which balance
is
obtained and
the
microphone sensitivity
is
50
mV
/Nfm2
(from calibration chart provided)
then
the
meter
deflection will be
1.25.
10-2.
50.
160
=+
100
mV
Note:
If
the Microphone
is
placed
160
mm above
the
balance
point
then
the
meter deflection will be
-100
mV.
Once calibration has been obtained
for
a microphone,
the
inconvenience
of recalibrating
that
particular microphone can be avoided by using
the
test
oscillator. The amplitude of
the
oscillator should c
be
adjusted _
to
provide
some convenient indication on
the
measuring
and
recording instrumentation
(up
to
146
dB
indication available with
the
1 inch condenser microphone
fitted).
For
later calibrations·
the
GAIN ADJ.
is
used
to
adjust
the
Test
signal
to
its reference level. The accuracy
of
the
test
oscillator
is
±
0.3
dB
for temperature variations
less
than
±
soc.
17
4.
APPLICATIONS.
The applications
of
the
B&K
Carrier Microphone System Type 2631
are
numerous
and
cover almost any field where measurements
of
slowly varying
air
pressures
and short duration
pulses
are
sought. Discussion
here
is
con-
fined
to
the sonic
bang
and
pressure
measurements.
SONIC BANG MEASUREMENTS.
Measuring Environment.
When
sonic
bang
measurements
are
to
be
taken
from
ground level,
an
open
space
reasonably free
from
obstructions
and
local undulations
is
re-
quired,
with
a hard surface in the form
of
a securely mounted, rigid baffle
(1.5 m diameter,
or
more) surrounding the measurement point.
If
obstruc-
tions
are
present then they should not subtend a solid
angle
greater than
0.004 steradians.
Free
air conditions apply when the microphone
can
be
mounted at suffi-
cient height and when obstructions in the upper half
space
do
not
subtend
an
angle
greater than 0.004 steradians. This may necessitate
an
altitude
of
more than 100 m
for
the whole sonic
bang
pressure
signature
to
be
deter-
mined. However, a lower height may
be
found sufficient
to
resolve the
rise
terms.
Any
deviations
from
the conditions specified
above
should
be
stated
with
the measurements obtained.
· Microphone & installation.
For sonic bang measurement when the upper limiting frequency
is
10kHz
or
greater, a
B&K
1/2 inch Condenser Microphone Type 4148
with
Head
Adaptor Type
UA
0271
is
recommended. This
is
to
avoid
measure-
ment errors due
to
the transit time
of
the shock
across
the microphone
diaphragm.
18
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